The goal of this proposal is to perform a comprehensive analysis of a novel form of resistance exercise training as a countermeasure to offset the loss in musculoskeletal structure and function after exposure to microgravity. Rats, continuously maintained in a hindlimb suspended state, will be subjected to resistance training using flywheel technology. We hypothesize that this form of resistance training, that is gravity-independent, will prevent muscle atrophy and loss of bone mineral density normally associated with hindlimb suspension. Force generated during training will be quantitated using a load cell and correlated with skeletal muscle mass, fiber cross-sectional area, and performance in Aim I. To identify specific mechanisms responsible for the changes in muscle properties in response to flywheel training, the dynamics of muscle protein homeostasis will be systematically quantitated. Aim 2 will explore the effects of flywheel training on bone mass, density and strength. Quantitation of the strain required for maintenance of bone properties using a strain gage will be correlated to muscle force to allow the development of a training regimen that is beneficial to the entire musculoskeletal system. The relative contribution of apoptotic muscle fiber nuclear loss and osteocyte apoptosis to loss of musculoskeletal function during hindlimb suspension and the effect of flywheel training will be determined in Aim 3. Finally, in Aim 4, muscle and bone marrow stem cells will be isolated and proliferation and differentiation analyzed to determine if changes in these cellular processes potentially contribute to the response to hindlimb suspension and flywheel resistance training. This approach will provide a basic understanding of mechanisms contributing to maintenance of the musculoskeletal system via resistance exercise during microgravity. Our experiments are directly relevant to studies being carried out in humans to determine efficacy of flywheel training protocols. Together, results may provide insight into the use of resistance exercise as a countermeasure to the loss of muscle and bone that occurs during space flight.